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<%
def exercise(title:, short_url:, start_code:, cheat_sheet:, solution:, long_url: nil)
<<-EOD.chomp
## Exercise: #{title}
`#{short_url}`
<!-- #{long_url} -->
## Exercise: #{title}
``` go
#{start_code}
```
## Exercise: #{title} cheat sheet
#{cheat_sheet}
`#{short_url}`
## Exercise: #{title} solution
``` go
#{solution}
```
EOD
end
%>
# Introduction to the Go Programming Language
## About me
* Author, _Head First Ruby_ and _Head First Go_
* 4 years experience as online software development instructor
* See my recent courses at `https://teamtreehouse.com`
## Where to Learn Go
`https://tour.golang.org`
![](images/go_tour.png)
(We'll repeat that link at the end.)
## Another humble recommendation
![](images/head_first_go_cover.png)
# Why Go?
## Go at a glance
* C-like syntax
* Compiles to native code
* Type-safe
* Garbage collected
* Concurrency built into language
## OK, but what can you do with Go?
## Docker
![](images/docker.png)
## Docker
* "'go build' will embed everything you need. (No more 'install this in order to run my stuff'.)"
* "Extensive standard library and data types."
* "Strong duck typing."
—Jérôme Petazzoni, "Docker and Go: why did we decide to write Docker in Go?"
::: notes
https://www.slideshare.net/jpetazzo/docker-and-go-why-did-we-decide-to-write-docker-in-go
:::
## Kubernetes
![](images/kubernetes.png)
## Kubernetes
* "Code in Go isn't overly complex. People don't create FactoryFactory objects."
* "Something with the feel of C with more advanced features like anonymous functions is a great combo."
* "Garbage Collection: We all know how to clean up after our selves but it is so nice to not have to worry about it."
—Joe Beda, "Kubernetes + Go = Crazy Delicious"
## Poll: What do you want to make with Go?
1. A system utility
1. A web app or service
1. Something else entirely
1. I don't know yet
## "go fmt"
* Automatically fixes code style
* Acts as community's style guide
* No more arguing tabs vs. spaces!
## "go fmt"
Before
``` go
package main
import "fmt"
func main() {
repeatLine("hello", 3 )
}
func repeatLine( line string ,times int) {
for i := 0; i < times; i++ {
fmt.Println(line)
}
}
```
## "go fmt"
``` go
$ go fmt repeat.go
```
## "go fmt"
After
``` go
package main
import "fmt"
func main() {
repeatLine("hello", 3)
}
func repeatLine(line string, times int) {
for i := 0; i < times; i++ {
fmt.Println(line)
}
}
```
## "go run"
* Compiles a Go source file and runs it
* No executable is saved
``` go
$ go run repeat.go
hello
hello
hello
```
## "go build"
* Compiles Go source file(s) into an executable
``` go
$ go build repeat.go
$ ls -l
total 2064
-rwxr-xr-x 1 jay staff 2106512 May 1 21:13 repeat
-rw-r--r-- 1 jay staff 166 May 1 21:13 repeat.go
$ ./repeat
hello
hello
hello
```
## Playground
* You don't even have to install Go to try it!
* Edit and run Go code in your browser
![](images/playground.png)
## Exercises for this training
* The Go Playground lets you save your code and share it at a URL
* Others can edit that code
* We'll use that ability for most of our exercises
* We'll post a link
* You visit it and follow the instructions there (add code, fill in blanks, etc.)
# Syntax
## Go file layout
* Package clause
* Imports
* Code
``` go
package main
import "fmt"
func main() {
fmt.Println("Hello, Go!")
}
```
## Calling Functions
``` go
package main
import "fmt"
func main() {
fmt.Println() // No arguments
fmt.Println("argument 1")
fmt.Println("argument 1", "argument 2")
}
```
## Calling Functions
* `fmt.Println` can take any number and type of arguments.
* Most functions require a _specific_ number and type of arguments.
``` go
package main
import "math"
func main() {
math.Floor(3.1415) // valid...
math.Floor() // not enough arguments
math.Floor(3.1415, 12.34) // too many arguments
math.Floor("a string") // wrong type
}
```
## Imports
``` go
package main
func main() {
fmt.Println(math.Floor(2.75))
fmt.Println(strings.Title("head first go"))
}
```
Compile errors:
```
prog.go:4:2: undefined: fmt
prog.go:4:14: undefined: math
prog.go:5:2: undefined: fmt
prog.go:5:14: undefined: strings
```
## Imports
Need to add `import` statement:
``` go
package main
import (
"fmt"
"math"
"strings"
)
func main() {
fmt.Println(math.Floor(2.75)) // => 2
fmt.Println(strings.Title("head first go")) // => Head First Go
}
```
## Unused imports not allowed
``` go
package main
import (
"fmt"
"os"
)
func main() {
fmt.Println("Hello, Go!")
}
```
Compile error:
```
temp.go:5:5: imported and not used: "os"
```
## "goimports"
* Wrapper for `go fmt`
* Automatically adds/removes imports
Install:
``` go
$ go get golang.org/x/tools/cmd/goimports
```
Do a web search for "goimports" for directions on integrating with your editor.
## "goimports"
Before saving
``` go
package main
func main() {
fmt.Println(math.Floor(2.75))
fmt.Println(strings.Title("head first go"))
}
```
## "goimports"
After saving
``` go
package main
import (
"fmt"
"math"
"strings"
)
func main() {
fmt.Println(math.Floor(2.75))
fmt.Println(strings.Title("head first go"))
}
```
## Variables
``` go
var myInteger int
myInteger = 1
var myFloat float64
myFloat = 3.1415
fmt.Println(myInteger) // => 1
fmt.Println(myFloat) // => 3.1415
fmt.Println(reflect.TypeOf(myInteger)) // => int
fmt.Println(reflect.TypeOf(myFloat)) // => float64
```
## Short Variable Declarations
``` go
myInteger := 1
myFloat := 3.1415
fmt.Println(myInteger) // => 1
fmt.Println(myFloat) // => 3.1415
fmt.Println(reflect.TypeOf(myInteger)) // => int
fmt.Println(reflect.TypeOf(myFloat)) // => float64
```
## Must use every variable you declare
``` go
subtotal := 24.70
tax := 1.89
fmt.Println(subtotal)
```
Compile error:
```
prog.go:9:2: tax declared and not used
```
## Type conversions
``` go
var length float64 = 1.2
var width int = 2
// Can't assign an `int` value
// to a `float64` variable:
length = width
fmt.Println(length)
```
Compile error:
```
cannot use width (type int) as type float64 in assignment
```
## Type conversions
``` go
var length float64 = 1.2
var width int = 2
// But you can if you do a type
// conversion!
length = float64(width)
fmt.Println(length) // => 2
```
## Type conversions
``` go
var length float64 = 1.2
var width int = 2
// Can't do a math operation with a float64 and an int:
fmt.Println("Area is", length*width)
// Or a comparison:
fmt.Println("length > width?", length > width)
```
## Type conversions
``` go
var length float64 = 1.2
var width int = 2
// But you can if you do type conversions!
fmt.Println("Area is", length*float64(width))
fmt.Println("length > width?", length > float64(width))
```
Output:
```
Area is 2.4
length > width? false
```
## "if"
``` go
if 1 < 2 {
fmt.Println("It's true!")
}
```
Output:
```
It's true!
```
## "if"
Parentheses discouraged. `go fmt` will remove these:
``` go
if (1 < 2) {
fmt.Println("It's true!")
}
```
Opening curly brace _must_ be on same line as `if`. This is a syntax error:
``` go
if (1 < 2)
{
fmt.Println("It's true!")
}
```
## "for"
``` go
for x := 4; x <= 6; x++ {
fmt.Println("x is now", x)
}
```
Output:
```
x is now 4
x is now 5
x is now 6
```
<%
@title = <<-'EOD'.chomp
Go syntax
EOD
@short_url = "https://is.gd/goex_syntax"
@long_url = "https://play.golang.org/p/x9BXw0z5LXT"
@start_code = <<-'EOD'.chomp
// Replace the blanks ("____") in the below code so that it
// compiles, runs, and prints the message "Hello, O'Reilly!".
____ main
____ "fmt"
____ main() {
myString ____ "Hello, O'Reilly!"
fmt.Println(____)
}
EOD
@cheat_sheet = <<-'EOD'.chomp
* Every Go source file is part of a __package__.
* To use code from other packages, you have to __import__ them.
* The `main` function is called when a program first starts.
* Functions are declared using the `func` keyword.
* A function call needs parentheses following the function name: `mypackage.MyFunction("my argument")`
EOD
@solution = <<-'EOD'.chomp
package main
import "fmt"
func main() {
myString := "Hello, O'Reilly!"
fmt.Println(myString)
}
EOD
%>
<%= exercise(title: @title, short_url: @short_url, start_code: @start_code, cheat_sheet: @cheat_sheet, solution: @solution, long_url: nil) %>
# Declaring Functions
## Declaring functions
``` go
func sayHi() {
fmt.Println("Hi!")
}
func main() {
sayHi()
}
```
## Function names
* Use `CamelCase`: capitalize each word after the first.
* If the first letter of a function name is `Capitalized`, it's considered __exported__: it can be used from other packages.
* If the first letter of a function name is `uncapitalized`, it's considered __unexported__: it can only be used _within_ its package.
* This is why all the names of standard library functions we've been calling are capitalized. (E.g. `fmt.Println`, `math.Floor`, etc.)
More on exported/unexported later.
## Parameters
``` go
// In parentheses, list parameter name(s)
// followed by type(s).
func say(phrase string, times int) {
for i := 0; i < times; i++ {
fmt.Print(phrase)
}
fmt.Print("\n")
}
func main() {
// Provide argument(s) when calling.
say("Hi", 4) // => HiHiHiHi
say("Bye", 2) // => ByeBye
}
```
## Variable scope
Variable scope limited to function where it's declared.
``` go
func myFunction() {
myVariable := 10
}
func main() {
myFunction()
fmt.Println(myVariable) // out of scope!
}
```
Compile error:
```
prog.go:11:14: undefined: myVariable
```
## Variable scope
By the way, variable scope also limited by "if" blocks:
``` go
if grade >= 60 {
status := "passing"
} else {
status := "failing"
}
fmt.Println(status) // out of scope!
```
## Variable scope
And by "for" blocks:
``` go
for x := 1; x <= 3; x++ {
y := x + 1
fmt.Println(y)
}
fmt.Println(y) // out of scope!
```
## Variable scope
Solution is to declare variable _before_ block:
``` go
var status string // declare up here
if grade >= 60 {
status = "passing" // still in scope
} else {
status = "failing" // still in scope
}
fmt.Println(status) // still in scope
```
## Variable scope
Same with loops:
``` go
var y int // declare up here
for x := 1; x <= 3; x++ {
y = x + 1 // still in scope
fmt.Println(y)
}
fmt.Println(y) // still in scope
```
## Function return values
So how do we get a value from a function to its caller?
``` go
func myFunction() {
myVariable := 10
}
func main() {
myFunction()
fmt.Println(myVariable) // out of scope!
}
```
## Function return values
Add a return value!
``` go
// Add return value type after parentheses
func myFunction() int {
// Use "return" keyword
return 10
}
func main() {
// Assign returned value to variable
myVariable := myFunction()
fmt.Println(myVariable)
}
```
## Multiple return values
``` go
func main() {
flag := strconv.ParseBool("true")
flag = strconv.ParseBool("foobar")
fmt.Println(flag)
}
```
Compile error:
```
prog.go:9:7: assignment mismatch: 1 variable but strconv.ParseBool returns 2 values
prog.go:10:7: assignment mismatch: 1 variable but strconv.ParseBool returns 2 values
```
## Multiple return values
``` go
func main() {
flag, err := strconv.ParseBool("true")
if err != nil {
log.Fatal(err)
}
fmt.Println(flag)
flag, err = strconv.ParseBool("foobar")
if err != nil {
log.Fatal(err)
}
fmt.Println(flag)
}
```
Output:
```
true
2009/11/10 23:00:00 strconv.ParseBool: parsing "foobar": invalid syntax
```
## Error handling
"In Go, error handling is important. The language's design and conventions encourage you to explicitly check for errors where they occur (as distinct from the convention in other languages of throwing exceptions and **sometimes** catching them)." (Emphasis mine)
-Andrew Gerrand, https://blog.golang.org/error-handling-and-go
## Writing functions with multiple return values
``` go
func myParseBool(myString string) (bool, error) {
if myString == "true" {
return true, nil
} else if myString == "false" {
return false, nil
} else {
return false, fmt.Errorf("bad string %s", myString)
}
}
func main() {
bool, err := myParseBool("false")
if err != nil {
log.Fatal(err)
}
fmt.Println(bool) // => false
}
```
## Writing functions with multiple return values
``` go
func myParseBool(myString string) (bool, error) {
if myString == "true" {
return true, nil
} else if myString == "false" {
return false, nil
} else {
return false, fmt.Errorf("bad string %s", myString)
}
}
func main() {
bool, err := myParseBool("foobar")
if err != nil {
log.Fatal(err) // => 2020-03-13 19:34:00 bad string foobar
}
fmt.Println(bool)
}
```
<!-- https://play.golang.org/p/Jih76DwKh4_s -->
<%
@title = <<-'EOD'.chomp
Declaring functions
EOD
@short_url = "https://is.gd/goex_define_functions"
@start_code = <<-'EOD'.chomp
package main
import (
"fmt"
)
// YOUR CODE HERE:
// Declare a "divide" function such that the call in the
// "main" function will compile and return 2.8.
// "divide" should accept two float64 values as parameters,
// and return a single float64 value that represents the
// first parameter divided by the second.
// EXTRA CREDIT:
// Have "divide" return TWO values, a float64 and an error.
// If the second parameter is 0, return an error value
// with the message "can't divide by 0". Otherwise, return
// nil for the error value. You can use the fmt.Errorf
// function to generate an error value. You'll also need
// to update the code in "main" to handle the error value.
func main() {
quotient := divide(5.6, 2)
fmt.Printf("%0.2f\n", quotient) // => 2.80
}
EOD
@cheat_sheet = <<-'EOD'.chomp
```
func oneReturnValue(param1 param1Type, param2 param2Type) returnType {
return valueToReturn
}
func twoReturnValues(param1 param1Type, param2 param2Type) (returnType1, returnType2) {
if thereIsAProblem {
return aMeaninglessValue, fmt.Errorf("an error message")
}
return valueToReturn, nil
}
```
EOD
@solution = <<-'EOD'.chomp
package main
import (
"fmt"
)
func divide(dividend float64, divisor float64) float64 {
return dividend / divisor
}
func main() {
quotient := divide(5.6, 2)
fmt.Printf("%0.2f\n", quotient)
}
EOD
%>
<%= exercise(title: @title, short_url: @short_url, start_code: @start_code, cheat_sheet: @cheat_sheet, solution: @solution, long_url: nil) %>
## Exercise: Declaring functions extra credit
``` go
package main
import (
"fmt"
)
func divide(dividend float64, divisor float64) (float64, error) {
if divisor == 0.0 {
return 0, fmt.Errorf("can't divide by 0")
}
return dividend / divisor, nil
}
func main() {
quotient, err := divide(5.6, 0.0)
if err != nil {
fmt.Println(err)
} else {
fmt.Printf("%0.2f\n", quotient)
}
}
```
## Pass-by-value
* Go is a "pass-by-value" language (as opposed to "pass-by-reference").
* This means Go functions receive a copy of whatever values you pass to them.
* That's fine, until you want a function to alter a value...
## Pass-by-value
``` go
func main() {
amount := 6
// We want to set "amount" to 12
double(amount)
fmt.Println(amount) // But this prints "6"!
}
// double is SUPPOSED to take a value and double it
func double(number int) {
// But this doubles the COPY, not the original
number *= 2
}
```
## Pointers
The `&` ("address of") operator gets a pointer to a value.
``` go
amount := 6
fmt.Println(amount) // => 6
fmt.Println(&amount) // => 0x1040a124
```
## Pointers
We can get pointers to values of any type.
``` go
var myInt int
fmt.Println(&myInt) // => 0x1040a128
var myFloat float64
fmt.Println(&myFloat) // => 0x1040a140
var myBool bool
fmt.Println(&myBool) // => 0x1040a148
```
## Pointers
A pointer to an `int` is written `*int`, a pointer to a `bool` as `*bool`, and so on.
``` go
func main() {
var myInt int
fmt.Println(reflect.TypeOf(&myInt)) // => *int
var myFloat float64
fmt.Println(reflect.TypeOf(&myFloat)) // => *float64
var myBool bool
fmt.Println(reflect.TypeOf(&myBool)) // => *bool
}
```
## Pointers
You can declare variables that hold pointers:
``` go
var myInt int
var myIntPointer *int
myIntPointer = &myInt
fmt.Println(myIntPointer) // => 0x1040a128
var myFloat float64
var myFloatPointer *float64
myFloatPointer = &myFloat
fmt.Println(myFloatPointer) // => 0x1040a140
```
## Pointers
The `*` _operator_ gets the value a pointer refers to.
``` go
myInt := 4
myIntPointer := &myInt
fmt.Println(myIntPointer) // => 0x1040a124
fmt.Println(*myIntPointer) // => 4
myFloat := 98.6
myFloatPointer := &myFloat
fmt.Println(myFloatPointer) // => 0x1040a140
fmt.Println(*myFloatPointer) // => 98.6
```
## Pointers
The `*` operator can also be used to update the value at a pointer:
``` go
myInt := 4
fmt.Println(myInt) // => 4
myIntPointer := &myInt
// Update the value at the pointer.
*myIntPointer = 8
fmt.Println(*myIntPointer) // => 8
fmt.Println(myInt) // => 8
```
## Pointers
We can use pointers to fix our `double` function:
``` go
func main() {
amount := 6
// Pass pointer instead of value
double(&amount)
fmt.Println(amount) // => 12
}
// Accept pointer instead of value